An onboard device such as a car navigator needs a technique for estimating the energy necessary to travel a given route. As a technique of calculating the energy necessary for traveling, resistances such as an air resistance, a grade (hill-climbing) resistance, a rolling resistance, and an acceleration resistance at the time of traveling are calculated and added. However, since the road grade or road surface state changes depending on the point, it is difficult to correctly grasp the value of the grade resistance or rolling resistance.
Hence, a technique of calculating a road grade by subtracting the energy consumed by accessories and an energy loss generated by traveling resistances other than the grade from the energy of an entire vehicle is used.